Lord Phillips, the country's most senior judge, said recently "the greatest concern for all involved in the criminal justice system must be the inexorable rise in prison numbers".

The number of people in prison in England and Wales, which has risen from about 66,000 in 2001 to more than 80,000 currently, was described by Phillips as having created "prisons bursting at the seams". These include about 2,500 children, described in a recent report by the Prisons Inspectorate and Youth Justice Board as "marginalised and excluded before they came into prison", with a third having been in care and the majority having been excluded from school.

"The situation with children in the prison system is an absolute disaster", said Phillippa Kaufmann, a human rights barrister at Doughty Street chambers, London.

"What I have seen over the years is that where children go into secure care settings and are provided with very intensive regimes including a substantial element of therapeutic work, one can see an amazing turnaround.

"It's a disaster that befalls if they enter the young offender estate, where they have to withstand the pressure on them and join gangs, take drugs, and do all the antisocial things that are the lifeblood of those institutions."

The increase in prison numbers is all the more unjustifiable because it is not related to an increase in crime, experts say. "Crime rates have been falling and, at the same time, sentences have become more severe", Phillips said, adding that many offences attracting short sentences could instead be dealt with in the community.

"Short sentences have an absolutely appalling effect on people", said Nick Armstrong, a barrister at Matrix Chambers and expert of prison law. "A six month sentence is not going to have a particularly punitive effect but is long enough to strip a man of the main factors that prevent future offending – home, job and relationship.

"The numbers of people in prison and delays moving them through the system plainly produces macro problems," ­Armstrong added. "You end up just ­needing more prisons".